1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) system, and more particularly, to a method for blind channel estimation using a guard interval of a signal in the OFDM technique.
2. Discussion of Related Art
In order to provide telecommunication services in various forms such as voice and packet data, wireless communication systems are widely disposed. The systems may be multiple access systems that share available system resources to support communication with multiple users. A code division multiple access (CDMA) system, a time division multiple access (TDMA) system, and an orthogonal frequency division multiple access (OFDMA) system are examples of multiple access systems.
Here, the OFDM is a technique that separates data into several bit streams and modulates the bit streams using several carriers. More specifically, the OFDM technique converts a serial bit stream into a parallel bit stream and modulates and transmits the information using subcarriers of different frequencies. In comparison with a method transmitting data in sequence using one carrier, the OFDM technique has a longer interval between transmission signals and thus is less affected by a channel delay time. Also, the OFDM technique can reduce interference between successive signals and thus is strong for a multipath channel. In addition, the OFDM technique can increase spectrum efficiency and shows good bandwidth efficiency because high-speed transmission is possible.
Therefore, the OFDM technique is hardly affected by time delay of a multipath and thus does not require a time-domain equalizer. In addition, the OFDM technique can eliminate interference between signals by inserting guard intervals.
In general, according to the OFDM technique, when there is no adjacent signal interference or adjacent channel interference caused by distortion of a transmission channel, orthogonality between subchannels is maintained, and each subchannel is completely separated by a fast Fourier transform (FFT) operation at a receiving end.
However, since a spectrum of an OFDM signal is not a limited band in actuality, energy of each subchannel is transferred to an adjacent channel due to linearity distortion such as multipath. Thus, adjacent signal interference is caused. While this problem can in theory be solved by increasing the number of carriers or a signal period, in practice this is difficult due to carrier stability, Doppler shift, and FFT size. Thus, instead, guard intervals are inserted into an OFDM signal.
FIG. 1 is a diagram illustrating a method for estimating a blind channel by inserting guard intervals into an OFDM system according to conventional art. Each OFDM symbol consists of two parts, i.e., an actual signal interval 10 and a guard interval 20. Here, the guard interval 20 is made by attaching an end part of a signal to a beginning part thereof.
Referring to FIG. 1, assuming that a k-th carrier in an i-th symbol block is si(k) in OFDM signal transmission, an i-th symbol block consisting of N carriers is expressed by a column vector si=[si(0), . . . , si(N−1)]T.
Here, each symbol si(k) is statistically uncorrelated with the others as in most known modulation methods (binary phase shift keying (BPSK), phase shift keying (PSK), quadrature amplitude modulation (QAM), differential phase shift keying (DPSK), and so on), independent of the others, identically distributed, and has a zero average and a variance of 1.
Here, in the OFDM system, the i-th symbol block si is modulated into a time-domain signal 10 by an N-point inverse fast Fourier transform (IFFT) operation. The time-domain signal 10 is expressed by the following Formula 1.ui=[ui(0), . . . , ui(N−1)]T=FNHsi  Formula 1
In Formula 1, FN denotes an N-point FFT matrix in which each element equals
      1          N        ⁢      ⅇ                  -        j            ⁢                          ⁢      2      ⁢      π      ⁢                          ⁢              nk        /        N            (n: row index, k: column index), and [•]H=([•])T)*.
Then, L guard interval samples 20 (prefix) are attached in front of an i-th OFDM symbol block ui by copy, and thus P (=L+N) number of OFDM symbol blocks ui,cp 30 are formed. Here, the samples within the guard intervals satisfy the following Formula 2.ui,cp(−L+n)=ui,cp(N−L+n), n=0, . . . , L−1  Formula 2
However, the OFDM system has come to use a pilot for channel estimation, resulting in deterioration of frequency band efficiency. For example, among channel allocation methods of Institute of Electrical and Electronics Engineers (IEEE) 802.16e, for which standardization has recently been completed, a partial usage of subchannels (PUSC) scheme uses one pilot per six carriers to obtain desired performance, thus deteriorating system efficiency.
Since the number of pilots increases in proportion to a desired throughput, frequency band efficiency consequently undergoes severe deterioration.
Therefore, blind channel estimation algorithms are proposed as a method for increasing frequency band efficiency, the blind channel estimation algorithms estimating a channel not by using a pilot but by using an inserted cyclic prefix within a guard interval repeated in OFDM symbol blocks.
The above-described blind channel estimation methods according to conventional art have the following problems.
First, the blind channel estimation methods that have been disclosed so far are very sensitive to channel zero and have unstable characteristics. Thus, stable, high-accuracy channel estimation is not possible.
Second, since a somewhat stable and accurate blind channel estimation method would be exceedingly complex, such a method would be difficult to practically apply to blind channel estimation.